The invention relates to hydrocarbon processing. In another aspect, the invention relates to the purification of natural gas liquids. In yet another aspect, the invention relates to an improved process employing an electrostatic precipitator for purifying a natural gas liquid stream.
In the processing of hydrocarbons, especially natural gas liquids, hereinafter "NGL", comtaminants often cause serious operating problems. Cryogenic gasoline plants frequently utilize materials such as methanol and glycol in their processes. Significant concentrations of methanol and glycol thus frequently exist in NGL pipelines.
Additionally, methanol is frequently injected as a dewpoint depressant into NGL pipelines, especially during the cold parts of the year. The purpose of the methanol is to prevent hydrate formation between certain components of the NGL and water, which is frequently also present as a contaminant in NGL pipelines. Furthermore, amines are frequently injected as corrosion inhibitors into NGL pipelines.
Naturally occuring contaminants in NGL pipelines include carbon dioxide and hydrogen sulfide. These materials are highly corrosive to pipelines constructed from materials containing iron, especially when water is present in the NGL as a cocontaminant. The hydrogen sulfide frequently attacks the interior surfaces of such pipelines forming particulate iron sulfide. The iron sulfide particles, a portion of which are in the micron size range are extremely difficult to remove from the NGL. Additionally, iron sulfide is pyrophoric, and when dry can present an ignition hazard around refineries and plants when exposed to oxygen-containing gas, such as air.
Iron sulfide additionally causes serious operating problems by fouling heat exchange equipment. The fouling results in reduced capacity and increased down time for cleaning.
Methanol and ethylene glycol when present in NGL feed are difficult to dispose of in an environmentally sound manner. In view of the fact that these materials frequently traverse NGL pipelines as slugs, waste water treaters must have very high design capacities for satisfactory operation. Furthermore, ethylene glycol and methanol interfere with the separation of CO.sub.2 and H.sub.2 S from NGL streams by amine treaters. For example, when the NGL is contacted with diethanol amine (DEA) solution, the glycol accumulates in the circulating DEA system. The subsequent dilution effect decreases DEA concentration, and both NGL treating and DEA regeneration are affected adversely. In addition, the increase in DEA solution inventory creates a serious disposal problem.
Heavy organic materials, such as gums and sludges, are also sometimes present as contaminants in NGL streams. These materials create problems by fouling filters and heat exchange equipment which results in decreased capacity and increased down time for cleaning.